Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 24)

Communication Concepts
Chapter 2
2-12
Generally, you need not be concerned with the interaction of station
interface modules on the network.
This means that your application programs at the PCs and computers
along the network need not involve themselves with interstation protocol,
handshaking, or control of the network link. This is all carried out
automatically by the station interface modules. However, an
understanding of station interaction is useful both to computer
programmers and PC programmers. It allows optimum use of network
commands and fault diagnostics.
Error Checking
Error codes can be generated at two places: remote station modules and
local station interface modules. For codes that are returned from a local
station module, two types of condition can exist:
Application programs use the wrong message format or issue illegal
commands.
The local station cannot complete a transaction due to network
problems.
A remote station can return only the codes associated with an application
problem at the remote station. Typically, these involve either the PC
processor being off-line (in Program mode, for example) or the command
trying to access memory areas blocked by either the interface module or
the user application program.
In the network layer protocol, command message status is returned in a
reply status byte. A value of zero in the status byte indicates successful
transmission of the corresponding command. It is up to the local
application program to display and act on the value of the returned byte.